Oxy-fuel combustion has received significant attention because it is widely considered to be a promising method for cost-effective CO2 capture. The key to the success of oxy-fuel combustion processes is the availability of inexpensive O2. Therefore, development of low-cost and robust technologies for air separation or separating O2 from air is critical. Great progress has been made in this research and development area. However, a recent review of the progress made in the area is lacking. This review is designed to fill the gap. The review focuses on three major technologies for separating oxygen from air, including membranes, adsorption, and chemical-looping with oxygen uncoupling. The separation principles, characteristics, challenges and the associated solutions of these major technologies are systematically discussed. Future opportunities are outlined for each.